http://en.wikipedia.org/wiki/AutismQuote:
Autism is a disorder of neural development characterized by impaired social interaction and verbal and non-verbal communication, and by restricted, repetitive or stereotyped behavior. The diagnostic criteria require that symptoms become apparent before a child is three years old.[2] Autism affects information processing in the brain by altering how nerve cells and their synapses connect and organize; how this occurs is not well understood.[3] It is one of three recognized disorders in the autism spectrum (ASDs), the other two being Asperger syndrome, which lacks delays in cognitive development and language, and pervasive developmental disorder, not otherwise specified (commonly abbreviated as PDD-NOS), which is diagnosed when the full set of criteria for autism or Asperger syndrome are not met.[4]
From what I recall, as with most psychological disorders, there is a list of symptoms, and typically something like 8 of 12 possible symptoms is sufficient for a diagnosis. So even if some autistic individuals do have problems with groups, not all of them necessarily will. Some children with autism exhibit a fascination and fixation with one particular thing while others may not. Some are severely overstimulated by one sense or another, while others are unaffected--its a poorly understood syndrome, with a lot of potential symptoms.
(For those of you with Netflix, try "Loving Lampposts" if you're curious about the subject.
My experience with autistic children is limited to 2 individuals (including the child that lives two houses down from me and is frequently put 'out to play' like any other child his age, though he is supervised.)